Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of the first image of a display screen or portion thereof with a transitional graphical user interface showing the new design;

FIG. 2 is a front view of the second image thereof; and,

FIG. 3 is a front view of the third image thereof.

The appearance of the image transitions sequentially between the images shown in FIGS. 1-3. The process or period in which an image transitions to another forms no part of the claimed design. The broken line showing of the display screen or portion thereof with a transitional graphical user interface, text and graphic elements forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 